This invention relates to a device and method for thermal processing a gas stream. More particularly, the invention relates to a spiral shaped thermal processor and a method of use thereof.
Developing efficient and stable combustors has been an object of longstanding research and development. Flame stability is often especially problematic for lean gas mixtures. A spiral wound combustor design, which may be used to combust lean gas mixtures, has particularly high heat recuperation or heat recirculation. The spiral wound combustor essentially consists of interspaced, spiral passages that lead to a central combustion chamber. One or more spiral inlet passages, which each lead to the central combustion chamber, are interspaced with one or more spiral outlet passages. Because the inlet gas is separated from the hot products of combustion only by the passage sidewalls, heat is transferred from the hot outlet gas stream to the incoming inlet gas stream, thereby preheating the incoming gas stream according to well known heat transfer principles.
Because of its spiral design and long residence time, spiral wound combustors generally are thermally efficient and capable of burning stoichiometrically fuel-lean mixtures while exhibiting stable combustion characteristics. A double spiral wound combustor is compact and especially effective at recuperating heat. (See for example Felix J. Weinberg, Advanced Combustion Methods, Chapter 3, page 207 (Academic Press, 1986) for an embodiment of a double spiral wound combustor). An example of a stoichiometrically fuel-lean mixture that may be processed in a spiral wound combustor is an off-gas containing hydrocarbons produced by various industrial processes. Such off-gases are generally regarded as atmospheric pollutants.
Despite such process-related advantages, spiral-type thermal processing devices have significant shortcomings. For example, spiral wound combustors typically are difficult to manufacture and maintain compared with simple, enclosed combustion chambers. For example, providing for uniform and desired spacing of the spiral passages during forming is often difficult. Maintenance is especially difficult at inner-most portions of the spiral wound combustor, which are most likely to exhibit the most severe wear and chemical attack because of the higher temperatures that often exist therein. Further, uneven temperature distribution may result in nonuniform thermal expansion of the sidewalls, endwalls that contain the sidewalls, structural supports, and auxiliary equipment. Uneven thermal expansion may create undesirable thermal stresses in combustor components, supports, and auxiliary equipment. For example, uneven thermal expansion may cause shell buckling or may promote sealing wear and failure. Furthermore, differential rates of thermal expansion during start-up and shut-down phases of operation exacerbate uneven thermal expansion problems relating to spiral wound combustors.
It is an object of the present invention to provide a spiral shaped device that enables flameless destruction of oxidizable gases (especially in lean mixtures), that produces a stable operation, and that has high heat recuperation.
It is another object of the present invention to provide a spiral shaped device that has a temperature distribution that diminishes local temperature gradients within the high temperature regions of the device so as to diminish uneven thermal expansion therein, to diminish uneven wear and chemical attack in such high temperature regions, and to diminish the formation of pollutants (such as oxides of nitrogen) associated with such high temperature regions.
It is another object of the present invention to provide seals for the device that maintain their effectiveness when the device thermally expands.
It is yet a further object of the present invention to provide a spiral shaped device having passages that are spaced apart by features on the sidewalls of the device. Such spacing features preferably also provide uniform spacing of the passages. Further, it is an object of the present invention to provide a spiral shaped device that has features for enhancing heat transfer on the sidewalls of the device.
It is yet a further object of the present invention to produce a device that provides efficient heat exchange between a gas stream and a fluid stream.
A spiral shaped device for thermally processing a gas stream and a method of use thereof is provided. The spiral shaped device is particularly useful as a recuperative flameless thermal oxidizer or as a heat exchanger. The spiral shaped device includes at least one sidewall defining a coil; at least one spiral passage defined by the sidewall and having an inlet and an outlet; an inlet assembly in flow communication with the inlet of the spiral passage; an outlet assembly in flow communication with the outlet of the spiral passage; and a matrix of heat resistant porous inert media disposed in at least a portion of the spiral passage.
In one embodiment of the present invention, the spiral shaped device includes a structure having at least two coiled sidewalls that are interspaced apart; at least two spiral passages formed between the two coiled sidewalls for passing the gas stream through the structure, where each coiled sidewall has an interior end and two longitudinal ends and at least one of the spiral passages is an inlet for the gas stream and at least another of the spiral passages is an outlet for the gas stream; means, located at the interior ends of the coiled sidewalls, for directing the gas stream in the at least one spiral inlet passage to the at least one spiral outlet passage; and a matrix of heat resistant porous inert media disposed in at least a portion of the structure.
When the device is used as a reactor, such as a flameless thermal oxidizer, the means for directing the gas stream preferably comprises a chamber, preferably located at the center of the structure and preferably containing matrix. The chamber is situated so that it is in flow communication with the spiral passage inlet and spiral passage outlet. In the case of a flameless thermal oxidizer, at least a portion of the gas stream is preferably oxidized in the chamber in a reaction wave.
When the device is used as a heat exchanger, the means for directing the gas stream is preferably at least one transition sidewall connecting the interior ends of two coiled sidewalls to form at least one loop that directs the gas stream from the spiral inlet passage to the spiral outlet passage. In a preferred embodiment, the heat exchanger contains at least four coiled sidewalls, at least four spiral passages, and at least one transition sidewall that connects the interior ends of two nonadjacent coiled sidewalls to form two loops, where the first loop directs a gas stream through the structure and the second loop directs a fluid stream through the structure.
In a preferred embodiment of the present invention the device has a plurality of dimples located on at least a portion of at least one of the coiled sidewalls . The dimples preferably protrude into at least one of the spiral passages. The dimples may also be used to space apart at least two of the coiled sidewalls and can also enhance heat transfer between the gas stream and the sidewalls.
In another preferred embodiment of the present invention, at least one coiled sidewall, and more preferably alternating coiled sidewalls, have an insulation layer. The insulation layer can be used to enhance overall heat transfer effectiveness of the device.
In yet another preferred embodiment of the present invention, the device comprises at least one seal that is in contact with at least one longitudinal end of the coiled sidewalls. Such a seal includes a compressible material that is disposed against at least one of the longitudinal ends of the coiled sidewalls. The compressible material may be rigidly fixed or biased against the coiled sidewalls. The seal has the advantage of being able to maintain tight contact with the longitudinal ends of the device while the device thermally expands.
The present invention also provides a method for thermally processing a gas stream that includes providing a device of the present invention; directing the gas stream to flow through the spiral inlet passage to the spiral outlet passage of the device; and thermally processing the gas stream in the device.
The present invention also provides a method for making a device of the present invention including providing at least two sidewalls; spirally winding the sidewalls; and forming dimples on at least a portion of a surface of at least one of the sidewalls, the dimples spacing apart the sidewalls to form at least two spiral passages therebetween.